1. Field of the Invention
The present invention relates to a method for forming contact window, and more specifically to a method that coating an over coating layer before forming contact window, where over coating layer has a higher etching rate than underlying layer, to let the contact window is outwardly widened.
2. Description of the Prior Art
Contact is broadly used in semiconductor fabrication to form metal plug, interconnect or other semiconductor structures. Formation of contact comprises: formation of a contact window on a surface, and fill material in the contact window to form the contact. Although the forming process of contact window comprises photolithography and etching, size of contact window is restricted with contemporary techniques, the restriction is not strict for even in deep-submicron fabrication. It is improved by application of deep UV, electron beam and X-ray, and then size of contact window is decreased as integration of integrated circuits (IC) is increased.
No matter how, although width of contact window is decreased as integration is increased, thickness of contact window is still restricted in a finite range for heights of related semiconductor elements still are not negligible. For example, typical thickness of contact window of logic IC is about 1 micron. Nevertheless, for memory IC there are both gate and capacitor and then typical thickness of contact window is about 2 micron to 2.5 micron. Thus, aspect ratio is increased as scale of semiconductor element is minimized and then step coverage of filled material is degraded.
An example of conventional contact is shown in FIG. 1A, where a coating layer 12 is formed over a surface of wafer 10 and then a contact window is formed by etching. Finally, material 14 is filled in the contact window.
Obviously, as aspect of contact window is increased, over hang 16 and void 18 are unavoidable and then qualify of contact is degraded. For example, if contact is a metal plug, high aspect may let the contact window is closed by over hang and then bottom of contact window is not filled by metal. Thus, the metal plug is not useful to conduct current.
A direct solution to reduce aspect is increasing width of contact width, especially while height of contact window is restricted by heights of semiconductor structures. By the way, because outline of contact window is a main factor that affects how material is filled in the contact window, another conventional contact window is shown in FIG. 1B, where coating layer 12 is formed on surface of wafer 10 and contact window is formed by anisotropic etching. Obviously, step coverage of material 14 is improved and there is no overhang or void if sidewall of contact window is enough lean. No matter how, allowable width of contact window is strongly limited by the increased integration. Because each contact must be isolated with other contacts or semiconductor elements, there is a lowest limitation of distance between each contact and other contacts or semiconductor element. Obviously, width of contact window is limited by specific critical dimension, and then aspect still is increased as thickness of contact window is increased.
For these foregoing reasons, obviously, it is important to form a contact without disadvantage such as void or overhang. In addition, it is urgent to solve the problem without obviously increase width of contact window, which is more important in the high-integrated semiconductor fabrication.
The present invention provides a method to form contact window. Essential concepts of this invention comprises: over coating layer is formed before the contact window is formed, where the etching rate of over coating layer is higher than etching rate of underlying structure. Thus, owing to it is more facile to remove over coating layer, the cross-section of contact window is wider in the over coating layer than the cross-section of contact window in the underlying structure.
First embodiment is a method for forming contact window. Provided method comprise following steps: First, form some semiconductor structures on surface of wafer and then form a coating layer surface of wafer, where thickness of coating layer is equal to or higher than height of semiconductor structures. Next, form an over coating layer over coating layer, where etching rate of over coating layer is higher than etching rate of coating layer. Then, form a contact window in both over coating layer and coating layer, where upper part of contact window is outwardly widened.
This embodiment at least could be optionally modified as following: planarize underlying structure before over coating layer is formed, form contact window by etching process, viscosity of over coating layer is larger than underlying structure; and upper part of contact window could be oblique, crooked, or smooth.